Iron Blueberries

The story of water on ancient Mars just got more interesting. Scientists working on the Mars Exploration Rover (MER) mission announced last week that the small spherules of rock, referred to as "blueberries," embedded in the bedrock outcrop near the Opportunity landing site contain the iron oxide hematite.

This graph shows two spectra, or light signatures, of outcrop regions near the Mars Exploration Rover Opportunity’s landing site. The blue line shows data for a region dubbed "Berry Bowl." The yellow line represents an area called "Empty" next to Berry Bowl that is devoid of berries. Berry Bowl’s spectrum still shows typical outcrop characteristics, but also exhibits an intense hematite signature, seen as a "magnetic sextet." These spectra were taken by the rover’s Mossbauer spectrometer on the 46th (Empty) and 48th (Berry Bowl) martian days, or sols, of its mission. Image Credit: NASA/JPL/Cornell/University of Mainz

Scientists previously announced that the rock matrix that makes up the bulk of the outcrop contained a high concentration of sulfate minerals – a clear indication, they said, that the rock was once saturated with water. Embedded within this rock matrix, and scattered across the floor of the crater, are tiny gray spheres about the size of BBs. Scientists earlier concluded that the spherules were "concretions," which form as groundwater moves through buried rock.

To determine what the spherules were made of, Opportunity drove to a spot on the outcrop where dozens of spherules had collected in a small depression, "Blueberry Bowl." The rover used its Mössbauer spectrometer to compare an area filled with spherules to one containing only matrix rock. Mössbauer spectrometers are designed to identify iron-bearing minerals, such as hematite.

Data from the Mössbauer spectrometer are used to generate a spectral graph, which looks like a squiggly line. By examining the pattern of peaks in a graph, scientists can identify specific iron-bearing minerals. The spectrum of the blueberry-rich area showed a set of six peaks that constitute a "fingerprint of hematite," said Daniel Rodionov, a graduate student at the University of Mainz in Germany and a member of the MER science team. "We can conclude that the major iron-bearing mineral in these blueberries is hematite." The hematite signature did not appear in the graph of the Mössbauer data collected when Opportunity pointed the instrument at the rock matrix.

Blueberries Everywhere

The discovery of hematite in the blueberries could provide a critical link that ties the history of the outcrop, Opportunity Ledge, to that of the surrounding Meridiani Planum region. It was the signature of hematite, detected from orbit by TES (Thermal Emission Spectrometer) aboard the Mars Global Surveyor (MGS) spacecraft, that originally piqued the interest of scientists in Meridiani as a landing site. Hematite typically forms in water.

Click image to enlarge.This false-color composite image, taken at a region of the rock outcrop dubbed "Shoemaker’s Patio" near the Mars Exploration Rover Opportunity’s landing site. Data from the camera’s infrared, green and violet filters were used to create this false-color picture. Image Credit: NASA/JPL

From orbit, hematite is in evidence over a vast region, covering some 180,000 square kilometers (about 70,000 square miles). Until the detection of hematite in the spherical concretions, however, no one was sure how the history of Opportunity Ledge related to that of Meridiani’s hematite.

That connection is still uncertain, but Harvard biologist and science team member Andy Knoll proposed an intriguing possibility. He speculated that "perhaps the whole floor of Meridiani Planum, this area about the size of Oklahoma," is covered with a "residual layer of blueberries." Knoll suggested that this blueberry layer, if it exists, might have produced the hematite signal that TES detected from orbit. "If that’s true then, then one might guess that a much larger volume of outcrop once existed and was stripped away by erosion through time," he said. The amount of water required to form hematite-bearing concretions throughout such a large rock formation would have been tremendous.

Images of the outcrop, taken by Opportunity’s Pancam, bolster Knoll’s hypothesis. "If you look above the outcrop, the whole surface is littered with berries. Now if those berries have the same origin as the ones which we actually see encased in the rock – and I think that’s a fair likelihood – then that tells us that once there was more rock there and that as the outcrop was eroded away, the resistant, hematitic berries accumulated," Knoll said.

Eagle Crater driving path. Image Credit: NASA/JPL

Once Opportunity completes its work in Eagle Crater, in about a week, it will drive up and out onto the surrounding plains to test this theory.

A Remaining Mystery

The new finding still leaves unanswered one important question about the bedrock outcrop: Did the bulk of the rock form in water, or did it form by some other process, and only later become altered by water? Some scientists speculate that the rock matrix is an evaporitic column that formed as, perhaps, a shallow lake or sea dried up, leaving behind a layered sequence of salt deposits.

Opportunity recently acquired new data from "Shoemaker’s Plaza," a finely layered section of the outcrop, that scientists hope will help them resolve this question. By looking for crossbedding among the layers – adjacent layers appearing at different angles – it should be possible to determine whether the layers are wind-blown deposits or were laid down under water, said MIT geologist John Grotzinger, also a member of the MER science team.

Rocks that exhibit crossbedding typically form through the action of surface water. If Opportunity Ledge formed at the martian surface, then Mars once must have been a much warmer place than it is today and its atmosphere must have been thicker. The results of the analysis of the rocks at Shoemaker’s Plaza are expected shortly.

Regardless of whether or not crossbedding is evident in the outcrop rocks, says Knoll, "these rocks were never deeply buried. You had to have water very near the surface." In any case, a tremendous volume of water was required to form the hematite-bearing spherules. Even water saturated with a relatively high concentration of iron, such as the water in Spain’s Rio Tinto, actually contains very little iron. So "the amount of water that would be needed to make those hematite nodules is many times the volume of the rock they’re in," said Knoll. On Earth, it would take a considerable volume of groundwater flowing through the rock, for "decades to centuries," to form them.